Purified cytochrome P-450 were examined for catalytic activity in the conversion of the (minus) and (plus) enantiomers of trans-7,8-dihydroxy-7,8-dihydrobenzo(a)pyrene to stereoisomeric, highly reactive and mutagenic 7,8-dihydroxy-9,10-oxy-7,8,9,10-tetrahydrobenzo(a)pyrenes. In the reconstituted enzyme system, P-450LM4 from both phenobarbital-and beta-naphthoflavone-induced animals has much higher catalytic activity than P-450LM2 with either enantiomer of the trans-7,8-diol. Both forms of the cytochrome exhibit greater activity toward the (minus) than the (plus) isomer of the substrate, but this is more striking with P-450LM4. The relative amounts of diol-epoxides formed from either enantiomer of the substrate differ markedly with the form of the cytochrome used. P-450LM2 gives somewhat more diol-epoxide I than diol-epoxide II with both substrates. In contrast, P-450LM4 gives almost exclusively diol-epoxide I from the (minus) trans-7,8- diol and more diol-epoxide II than diol-epoxide I from the (minus) trans-7,8-diol. Thus, P-450LM4 is highly stereospecific in oxygenating at the 9,10 double bond, regardless of the absolute configuration of the hydroxyl groups at the 7 and 8 positions of the substrate.